1. Field of the Invention
The present invention relates to fixed bed catalysts and their use for the hydrogenation of saturated and unsaturated esters.
2. Description of the Background
Activated metal catalysts are known in the field of chemical engineering as Raney catalysts. They are used, largely in powder form, for a large number of hydrogenation, dehydrogenation, isomerization reductive alkylation, reductive amination, and hydration reactions of organic compounds. These powdered catalysts are prepared from an alloy of a catalytically active metal, also referred to herein as a catalyst metal, with a further alloying component which is soluble in an alkali. Suitable catalytically active metals include nickel, cobalt, copper and iron. Aluminum is generally used as the alloying component which is soluble in an alkali, but other components may also be used, in particular, zinc or silicon or mixtures of either one of these elements with aluminum.
Powdered catalysts have the disadvantage that they can be used only in a batch process and, after the catalytic reaction, have to be separated from the reaction medium by costly sedimentation and/or filtration techniques. Therefore, a variety of processes for preparing molded forms of the catalysts, which lead to activated metal fixed-bed catalysts after extraction of the aluminum have been disclosed. Thus, for example, coarse particulate Raney alloys, i.e., Raney alloys which have only been coarsely milled, can be prepared and these alloys can be activated by treatment with a caustic soda solution. Extraction and activation then occurs only in a surface layer, the thickness of which can be adjusted by the conditions used during extraction.
A substantial disadvantage of catalysts prepared by these known methods are the poor mechanical stability of the activated outer layer. Since only this outer layer of the catalysts is the catalytically active component, abrasion leads to rapid deactivation and renewed activation of deeper lying layers of alloy using caustic soda solution then leads at best to partial reactivation.
It is known that Re doped Pd (DE 25 19 817 A1)or Re doped Ru (WO 96/27436) supported catalysts are useful for the production of gamma-butyrolactone (GBL), tetrahydrofuran (THF), and 1,4-butanediol (BDO) by either maleic acid or maleic anhydride hydrogenation. These systems work as either powder or fixed bed catalysts at pressures of 138 bar or higher and temperatures of 250xc2x0 C. or higher. In this respect, a system that performs this reaction at milder conditions would be a strong advantage. Copper-chromite is another catalyst system that has used for ester hydrogenation with some success (DE 39 03 029 A1).
EP 0 648 534 A1 describes shaped, activated Raney metal fixed-bed catalysts (Metalyst(copyright)) and their preparation. These catalysts avoid the disadvantages described above, i.e., the poor mechanical stability resulting from activating an outer layer. To prepare these catalysts, a mixture of powders consisting of a catalyst alloy and a binder are used, where the catalyst alloy contains at least one catalytically active catalyst metal and an extractable alloying component. The pure catalyst metals or mixtures thereof, which do not contain an extractable component, are used as the binder. The use of the binder in an amount of 0.5 to 20 weight percent with respect to the catalyst alloy, is essential in order to achieve sufficient mechanical stability after activation. After shaping the catalyst alloy and the binder with conventional shaping aids and pore producers, the freshly prepared items which are obtained are calcined at temperatures below 850xc2x0 C. As a result of sintering within the finely divided binder, solid compounds are produced between the individual granules of the catalyst alloy. These compounds, in contrast to catalyst alloys, are non-extractable or only extractable to a small extent so that a mechanically stable structure is obtained even after activation without endangering the strength of the shaped item.
Accordingly, one object of the present invention is to provide fixed bed activated base metal catalysts that hydrogenate saturated and unsaturated esters under milder conditions than existing technologies with better activities and selectivities.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a shaped, activated Raney metal fixed-bed catalyst prepared by a method comprising preparing a mixture of powders consisting essentially of at least one catalyst alloy of (1) at least one catalytically active Raney process metal, a leachable alloy component and optionally a promoter, (2) at least one binder containing at least one pure Raney process metal, and (3) a moistening agent, and optionally an additive selected from the group consisting of a shaping aid, lubricant, plasticizer, pore-producer, and mixtures thereof; homogenizing the mixture; shaping the mixture into a molded catalyst precursor which is not activated; calcining the molded catalyst precursor at a temperature below 850xc2x0 C. to prepare a sintered catalyst precursor, and activating the sintered catalyst precursor by leaching the leachable alloy component with alkali until the leached and thereby activated outer layer has an adjustable thickness corresponding up to 70% or more of the of the molded form being activated, and subsequently washing the final catalyst; doping said catalyst with rhenium as a promoter after said activation and washing by introducing said catalyst into a rhenium solution. The pH of the Re solution may or may not be adjusted, and the temperature of the doping solution may vary from lower than room temperature to substantially higher temperatures. Moreover, the rhenium may be added to the unactivated alloy, the binder, or introduced in any other fashion that allows for its presence in the catalyst. The Re content can range from 0.01% Re to 30% Re, preferably from 0.01% to 15%, more preferably from 0.01% to 10% by weight of the catalyst.
The fixed bed catalysts of the invention have the advantage that Re doped metal catalysts promote the hydrogenation of maleic acid or maleic anhydride to gamma-butyrolactone, tetrahydrofuran or 1,4-butanediol at a temperature of 200xc2x0 C. and a pressure of 80 bar. Additionally the present catalyst is able to hydrogenate fatty esters to the corresponding fatty alcohols at higher activities and selectivities than the standard copper chromite catalysts.
Preferred Raney process metals include nickel, cobalt, copper, or combination thereof and the leachable alloying components include aluminum, zinc, silicon, or combinations thereof. These metals are generally leached by an alkali such as NaOH. The ratio by weight of Raney process metal to leachable alloying component in the catalyst alloy is in the range from 10:90 to 90:10, as is normally the case with Raney alloys. The Raney process metal used as binder, in a real practical application, does not have to be the same as the catalyst metal present in the catalyst alloy. Rather, it is possible to combine different Raney process metals with each other as well as with promoter metals, in the catalyst alloy and as binder, offering a further important degree of freedom when adjusting the catalytic properties to the particular catalytic process. Thus the binder employed in the present invention can be nickel, cobalt, copper, iron, and optionally promoter metals. Generally any of the metals used for making Raney metal catalysts are suitable. The binder metal is employed in an unreachable and unadulterated form. Catalyst alloy and binder are processed in the form of powders, typically with the addition of moistening agents and optionally with the addition of conventional additives such as shaping aids, lubricants, plasticizers, and optionally pore-producers to give a moldable material. Any materials conventionally used for these purposes may be used as the shaping aid, lubricant, plasticizer and pore-producer. A number of suitable materials for this purpose are disclosed in U.S. Pat. Nos. 4,826,799; 3,404,551; and 3,351,495 all of which are incorporated by reference in their entirety. Waxes such as, for example, wax C micropowder PM from Hoechst AG, greases such as magnesium or aluminum stearates, or polymers which contain carbohydrates such as tylose (methylcellulose) are preferably used for the above purposes.
The solids in the mixture are carefully homogenized in suitable conventional mixers or kneaders with the addition of a moistening agent. Water, alcohols, glycols, polyether glycols or mixtures thereof are suitable as moistening agents as is well known in the art.
The primary particle size ranges of the powders of catalyst alloy and binder used are essentially unchanged during homogenization. That is, no milling takes place.
The purpose of this preliminary treatment with the moistening agent and additives is to prepare the mixture for the subsequent shaping process. Extrusion, pelleting and compression may be used, for example, for the shaping process employing conventional equipment known for such purposes.
The type and sequence of incorporation of additives depends on the shaping process to be used. Extrusion requires a plastic material with a specific viscosity, whereas a material which is free-flowing and which can be readily metered out is required for pelleting. The techniques to be used for this purpose, such as, for example, agglomeration to produce a free-flowing powder or adjustment to the correct viscosity for extrusion, are known as a matter of routine to the person skilled in the art. It is only important that the primary particle size ranges of the catalyst powder and binder powder are essentially unchanged by the preliminary treatment.
The molded structures are optionally dried to constant weight at temperatures ranging from 80xc2x0 C. to 120xc2x0 C. and then calcined at temperatures below 850xc2x0 C., preferably from 500xc2x0 C. to 700xc2x0 C., in air in continuous or batch operated kilns such as rotary kilns or stationary kilns. The organic additives then bum off and leave behind a corresponding porous system.
The porous structure and pore volume of the catalysts can be varied over a wide range by suitable selection of the pore-producing additives. The final pore structure which is developed and the pore volume are also affected by the particle sizes of the powders of catalyst alloy and binder employed.
The structure of the molded catalyst can be adapted to the requirements for a particular catalytic process by appropriate selection of the parameters mentioned.
During calcination of the molded catalyst structures, the catalyst alloy powder and binder powder sinter together and provide the molded catalyst structures with high mechanical stability and good resistance to abrasion. Typically, the hardness of cylindrical pellets after calcination ranges from 200 to 300 N (measured radially in accordance with ASTM D 417982).
After calcination the molded catalyst structures are activated by leaching the aluminum with caustic soda solution. A 20% strength sodium hydroxide solution warmed to 80xc2x0 C. can be used for this purpose. In this case, treatment for 2 hours leads to an active outer layer with a thickness of about 0.1 to 1 mm. Surprisingly, it has been shown that the hardness is actually slightly increased by leaching, in the case of pellets to values of more than 300 N.
These properties are closely connected with the pure Raney process metal employed as binder which is not dissolved during leaching and thus in the sintered product forms stable bonds between the individual alloyed particles. According to German Patent DE 197 218 98.9 (Freund, Berweiler, Bender and Kempf, 1998) the use of a metallic binder for the production of Metalyst(copyright) can be avoided if the phase domains of the alloy are sufficiently small enough. The size of the phase domains can be controlled by the cooling rate and method employed for cooling of the alloy. Hence, the use of a binder in the invention of this patent is optional and the technology of DE 197 218 98.9 is applicable to this catalyst.
The choice of metals used as binder may contribute to the catalytic activity. Restricting the temperature of calcination to values below 850xc2x0 C. prevents the formation of alpha-aluminum oxide as shown by X-ray diffraction analysis of the calcined material. Any xcex3-aluminum oxide which is formed is removed by dissolution from the catalyst structure when activating the catalyst with caustic soda solution.
The lack of xcex1-aluminum oxide in the catalyst becomes clearly noticeable on activation. Whereas catalysts of the present invention can be activated under quite mild conditions (20% NaOH, 80xc2x0 C.) within only 2 hours, the temperature of the alkaline solution has to be raised and the activation time extended when activating catalysts bonded with xcex1-aluminum oxide (according to U.S. Pat. No. 4,826,799) in order to obtain an active outer layer of the same thickness.
To prepare the catalyst of the present invention, the average particle sizes of the catalyst alloy powder and of the binder, and also the ratio by weight of catalyst alloy powder to binder, can be varied over a wide range. Since the binder also contributes to the catalytic activity, but it cannot be activated by extracting aluminum, its possible contribution to the catalytic activity is limited. Therefore, its proportion in the catalyst should be kept as small as possible.
Ratios by weight of catalyst alloy powder to binder range from 100:20 to 100:0.5 have proven to be useful. The particle size of the binder should be smaller than the particle size of the catalyst alloy powder. Particles of binder can then be regarded as small bridges between the larger alloyed particles. It has been found that the hardness of the final catalyst structure increases within certain limits with decreasing particle size of the binder. Reasonable activity values are obtained when the powder of the catalyst alloy has an average particle size ranging from 10 and to 500 xcexcm.
When the catalyst is doped with rhenium, it is expedient to conduct doping only after activating the catalyst. For this, the final catalyst is introduced into a rhenium solution, e.g., perrhenic acid. The amount of rhenium and the time needed for its addition can be controlled by adjusting the pH and the temperature of the rhenium solution. A specific amount of the rhenium compound is adsorbed by , the catalyst, depending on the type of treatment, e.g. up to 20% by weight.
An aspect of the invention is a process for preparing the shaped Raney metal fixed-bed catalyst. The process consists essentially of preparing a mixture of powders consisting essentially of at least one catalyst alloy and optionally one binder, and a moistening agent. An additive selected from the group consisting of a shaping aid, lubricant, plasticizer, pore-producer, and mixtures thereof, is optionally included, and said catalyst alloy consists essentially of at least one Raney process metal as the catalytically active component, a leachable alloy component and optionally a promoter. The optional binder consists essentially of at least one Raney process metal. Processing continues by homogenizing the mixture, shaping the mixture to give a molded catalyst precursor which is not activated, calcining the molded catalyst precursor at a temperature less than 850xc2x0 C. to prepare a sintered catalyst precursor, and activating the sintered catalyst precursor by leaching the leachable alloy component with alkali until the leached and thereby activated outer layer has a thickness of 0.05 to 1 mm or higher; optionally subsequently washing the final catalyst. Rhenium, after activating and washing the catalyst is added by introducing the catalyst into a rhenium solution, where rhenium deposition is controlled by the temperature and pH of the rhenium solution.
Another aspect of the invention is the use of the rhenium doped Raney metal fixed-bed catalyst for the hydrogenation of unsaturated and saturated esters. An example of such a process is the hydrogenation of maleic anhydride to xcex3-butyrolactone, tetrahydrofuran, 1,4-butanediol under mild conditions. Moreover, it has been found that the present catalyst is an excellent catalyst for the hydrogenation of fatty esters to fatty alcohols.